A number of imaging methods and techniques have been used to visualize blood vessels in vivo but have demonstrated limitations pointing to a need for a new way to produce better results.
For example, a state-of-the-art MRA method, the maximum intensity projection (MIP), may not accurately show areas where blood vessels overlap in the 2D projection. On the other hand, volume rendering method of MRA data might depict such overlap more accurately but at the expense of overall reduced contrast-to-noise ratio (CNR) that can obscure small vessels and the expense of the need to adjust two independent variable parameters and thus make routine clinical usage difficult.
A much earlier method of imaging blood vessels in vivo is conventional x-ray angiography, in which a contrast agent is injected into the patient's arterial system and x-ray images are acquired that show blood vessels as translucent high-intensity areas. Where two vessels overlap, the image is brighter than for the individual vessels. Seeing through vessels and being able to identify overlapping vessels permits an analysis to determine relationships and origins of branching vessels and depth of vessels in the body and thus enable effective spatial interpretation of the x-ray arteriogram.
Angiographic images created by MRI techniques can show vessels in a patient's body but may not compare favorably with conventional x-ray angiograms in terms of contrast relative to the surrounding tissue and contrast-to-noise ratio (CNR), thus making it difficult in some cases to distinguish blood vessels from the surrounding tissue. MRA is a 3D technique, as opposed to 2D x-ray angiography, but the 3D information is not visualized directly so a projection method is used to produce 2D MRA information. First, the 3D information is assembled from many MRI slices, which are essentially 2D images, by piecing them together with known software to form a 3D set of data representing the volume of interest in the patient. Typically, the blood vessels in this volume are represented by the more intense pixels (or voxels). However, their intensity is not much greater than that of the surrounding tissue.
One way to better separate data representing blood vessels from data representing surrounding tissue is to use the maximum intensity projection (MIP) method, which has a particularly high CNR. (See Ref. 1 cited in the endnotes and note that the contents of each reference cited in the endnotes are hereby incorporated by reference in this patent specification.) This method creates a projective 2D image of the vessels contained in the volume represented by the 3D data. To construct the 2D image, the MIP method forms a set of notional (imaginary) rays extending through the 3D data, with a respective ray for every pixel in the 2D image and with each data point (for pixel or voxel) in the 3D image being intersected by only a single ray. For a given ray, the MIP method selects the single most intense data point that the ray intersects and uses that data point to control the brightness (value) of the pixel in the 2D image associated with that ray. This process continues to create a 2D image composed of the pixels associated with all the rays. Methods to further improve CNR and the signal-to-noise ration (SNR), and thus improve the visualization of blood vessels, have been proposed. See Refs. 2, 3.
For example, the 3D MRA information may comprise 256×256×256 data points that represent a corresponding 3D array of pixels or voxels in the patient's body. Of course, the data point array need not be stored as a 3D array so long as the relationship between a data point and its 3D position can be ascertained when needed. The projective 2D image representing the 3D array may contain 256×256 pixels. To construct the 2D image, 256×256 notional rays, one for each pixel in the 2D image, are extended through the 3D array, with each ray notionally intersecting 256 data points in the 3D array for the example of non-rotated projections. For each ray, the single most intense data point of the 256 points the ray intersects is selected, and the value of this single data point from the 3D array is used to control the display of the associated pixel in the 2D image. Proposals have been made to refine this general approach to MIP but are not believed to have found widespread clinical applications, probably due to CNR limitations. See Refs. 4–6.
The MIP method has a number or limitations. One is that only one of the (256) data points of the 3D array that a ray intersects is used in constructing the 2D image. In this example 255 out of 256 data points are not used in the construction of the 2D image except to the extent of identifying the one point to be used. Important details and valuable information can be lost by not making more use of the vast majority of the information in the 3D array. For example, if two vessels overlap each other in the 2D projection, the MIP method will use information from only one of the vessels, not from both, to show the overlap area in the 2D image, because only one data point from the 3D array can be selected for any one ray. Only the vessel in the foreground or the vessel in the background will be shown but not both. This can lead to misrepresentation of relative positions of vessels in the 2D image. It also causes a generally flat appearance of vessels in the MRA image produced by the MIP method as many of the depth perception visual clues are missing from the image.
One improvement on the MIP method is referred to as net intensity projection (NIP) and is discussed in a provisional application filed September 1998 by Koichi Oshio, entitled “Method and Apparatus for Projecting MR Angiographic Data”. Instead of selecting only the single most intense data point in the 3D array that an imaginary ray intersects, the NIP method selects all of the more intense points, i.e., all the data points above a selected intensity threshold. The pixel value in the 2D image that corresponds to this ray is calculated by adding up the values of the selected points from the 3D set, with appropriate normalizing if desired. Because typically several data points from the 3D set are used to represent a vessel pixel in the 2D image, instead of the single point for a MIP image, the NIP method produces a 2D image that can give visual clues about the 3D structure of the vessels. Overall appearance of the projection image can be more akin to conventional angiograms. Larger blood vessels can look more intense than smaller vessels, and overlapping portions of blood vessels running toward the observer can have higher intensity. Since background data points in the 3D image typically have lower intensity, most data points from the 3D image that correspond to non-vessel tissue are not added in when calculating the pixel values for the 2D image. Therefore, it is possible to maintain good contrast between vessels and background tissue. However, compared to the MIP method, the 2D images from the NIP method can make it more difficult to visualize small vessels.
Another known method of inspecting the 3D data set is to use 3D rendering techniques. They are generally divided into two classes, surface rendering (SR) and volume rendering (VR). SR involves a geometric reconstruction of the scalar valued 3D MRA data set using triangular meshes for example. The geometric representation can be visualized by known 3D graphic algorithms including color, shading and additional surface properties. Since for this purpose the surface properties of the blood vessels have to be detected first, this method tends to be time consuming and also error-prone for noisy data. Methods have been proposed to post-process MRA data but are not believed to have achieved much success. See Refs. 7–9. Volume rendering (VR) creates 2D images from the 3D data set by taking into account all or most of the 3D data points, unlike MIP. However, the vast majority of the data points in the 3D set represents tissue other than the blood vessels of interest. When the many data points representing other tissues are combined with the relatively few points representing vessels, they can overwhelm the information of interest. The resulting image may contain details showing how blood vessels overlap at different depths but other details can be washed out due to the inclusion of surrounding tissue. For example, in displaying small blood vessels the CNR can be greatly reduced in the VR method as compared with the MIP method. Parameters such as the values of data points in the 3D array can be attenuated if they are likely to represent tissue other than blood vessels. In addition, VR suffers from the difficulty that the parameters of shading and intensity must be selected and adjusted by the operator. These two parameters are not directly related to the familiar “window” and “level” with which MRI technologists are experienced. For these and other reasons, the robustness and consistency of creating and displaying VR images compared with the MIP method is a problem that has limited the acceptance and utilization of VR for this application. The longer operator time required to obtain a satisfactory result with VR compared with MIP can be a significant economic issue given the expense of additional scanner time and operator time.
To overcome these and other limitations, a new approach has been developed, called “translucent intensity projection” (TIP) in this patent specification. TIP makes a particularly efficient use of relevant information from the 3D data set but removes undesired effects of unneeded or deleterious information. The TIP approach can allow the operator to dial in as much 3D information as desired, in a way that can be conceptualized as a synergistic combination of MIP and NIP features. TIP can produce significantly superior results while avoiding issues presented by the SR and VR methods.
The TIP method both achieves high sensitivity (CNR and SNR), a characteristic present in MIP but absent from NIP images, and provides 3D visual clues in the 2D image, a characteristic present in NIP but absent from MIP images. This gives TIP images high image contrast between small vessels and the background but also 3D visual clues. Whereas MIP applies 100% weight to the most intense data point in the 3D image that a ray intersects and 0% weight to all other points this ray intersects, and NIP applies the same weight to the set of N more intense points that the ray intersects, TIP applies a high weight to the point that MIP would pick and lower but still significant weights to adjacent or nearby points along the ray path. Some or all of these points may also have been picked in NIP, but NIP applies the same weight to all picked points, including the point that MIP would have picked.
An additional improvement disclosed in this patent specification is called “local net intensity projection” (LNIP) and involves using for the 2D projection more of the 3D points that MIP but less than NIP, and selects the points to be used in a way designed to show small vessels and also retain visual clues on vessel depth in the body.